In recent years, requirement for low fuel consumption is becoming severer in connection with a movement of global emission control of carbon dioxide which follows social requirement for energy saving and a rise in concern about environmental problems. In order to meet the above requirement, tires which are reduced in rolling resistance and have a low heat build-up as a tire performance have come to be demanded. While a method for optimizing a tire structure has so far been studied as a method for reducing rolling resistance of a tire, it is carried out as the most usual method to use a material exhibiting a lower heat build-up as a rubber composition.
It has so far been carried out as a method for obtaining the above rubber composition having a low heat build-up to improve reinforcing fillers and rubber components.
Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black can provide rubber compositions with high abrasion resistance. When a low heat build-up is to be achieved by using carbon black alone, it is considered that a compounding amount of carbon black is reduced or that carbon black having a large particle diameter is used, but in both cases, it is known that abrasion resistance and grip on a wet road are not prevented from being reduced. On the other hand, it is known to use silica as a filler in order to enhance a low heat build-up (for example, patent documents 1 to 4). However, particles of silica tend to be aggregated by virtue of a hydrogen bond of a silanol group which is a surface functional group of silica, and silica is inferior in wettability with rubber molecules, so that dispersion of silica into rubber is not good. The kneading time has to be extended in order to improve the above problem. Also, if silica is insufficiently dispersed into rubber, a rubber composition is elevated in a Mooney viscosity and inferior in processability such as extrusion. Further, since a surface of a silica particle is acidic, silica adsorbs a basic substance used as a vulcanization accelerator in vulcanizing a rubber composition to prevent the rubber composition from being sufficiently vulcanized, so that the defect that the elastic modulus is not enhanced has been involved therein.
A silane coupling agent has been developed in order to improve the above defects, but dispersion of silica has not yet reached a sufficiently high level, and particularly it has been difficult to obtain industrially good dispersion of silica particles. Accordingly, it is tried to mix silica treated a surface thereof with a hydrophobicity-providing agent to accelerate reaction of a silane coupling agent (patent document 5).
Further, it is disclosed in a patent document 6 to use hydrophobic precipitated silica. However, since precipitated silica subjected to complete hydrophobicity treatment is used, surface silanol groups to react with a silane coupling agent disappear, and therefore the defect that rubber is not sufficiently reinforced has been involved therein. Further, silica having an increased particle diameter is used in order to improve a low heat build-up, but in that case, silica is reduced in a specific surface area by increasing a particle diameter to deteriorate a reinforcing property thereof. It is disclosed in patent document 7 to use silica having a specific form, but a low heat build-up and abrasion resistance of the rubber composition are not sufficiently high.
On the other hand, many modified rubbers interacting with fillers such as silica, carbon black have been developed as methods for improving rubber components. Among them, particularly a method in which a polymerization-active end of a conjugated diene base polymer obtained by anionic polymerization using an organolithium compound is modified with an alkoxysilane derivative having a functional group interacting with fillers is proposed as the effective method (for example, patent documents 8 and 9).
However, many of them are applied to polymers in which a living property of a polymer end can readily be secured, and methods in which a modification effect in a rubber composition compounded with silica and carbon black is sufficiently exerted are not necessarily obtained. Also, in many of conventional modifying methods, principal chains can not sufficiently be provided with branches, and therefore they have involved therein the problems that cold flow is a large obstacle when they are put into actual use and that when partial coupling is carried out in order to deal with the obstacle, the modification effect is inevitably reduced.
Accordingly, a method in which a condensation accelerating agent is added to a reaction system in order to further enhance a modification effect in modifying an active end of a conjugated diene base polymer with an alkoxysilane is proposed (for example, patent document 10). In the rubber composition, a low loss effect thereof to silica base fillers is enhanced to a large extent, but the low loss effect to carbon black is not necessarily satisfactory.